1. Field of the Invention
The present invention relates to the field of power electronics. It concerns a liquid cooling device for a high-power semiconductor module, which contains a plurality of heat-generating submodules arranged next to one another on a cooling surface, each of the submodules comprising at least one power semiconductor device applied to an electrically insulating ceramic substrate and being fusion-bonded to the cooling surface on the underside of the ceramic substrate, and which liquid cooling device (1) has a housing (2) which encloses a liquid space (4), through which a cooling liquid flows, and the upper side of which forms the cooling surface (3).
2. Discussion of Background
High-power semiconductor modules are components for power electronics. A module generally contains a plurality of semiconductor devices, which may be combined to form a logical functional unit. Such modules (for example thyristor, IGBT or diode modules) are popular nowadays in the power range of up to 2500 V and a few 100 A and are used in particular in industrial drives. Examples of modules of the type mentioned are known from EP-A1-0 597 144 or from an article by T. Stockmeier et al., Reliable 1200 Amp 2500 V IGBT Modules for Traction Applications, IEEE IGBT Propulsion Drives Colloquium, London, April 1995.
So far, these modules have been used only to a very limited extent in traction drives. One of the reasons for this is the long-term reliability required, something which conventional modules have not so far provided. With the current state of the art, an important failure is the fatigue of the solder layers between the substrate on which the silicon chips are mounted and the water or liquid cooler connected to the module. This finally leads to overheating of the chips and to detachment of the electrical contacts. The effect is observed after a number of load cycles in which the chips are heated up to the maximum permissible operating temperature by means of the self-generated heat dissipated during operation and then cooled again to the temperature of the cooling water. The maximum number of load cycles after which the failure mentioned occurs depends here strongly on the temperature of the cooler, the temperature swing and the rate of the temperature change. Altogether, the achievable load cycle resistance is much too small for the requirements in the traction field to allow reliable use.
DE 42 44 721 A1 specifies semiconductor valves for electrical machines which have an integrated fluid cooling arrangement. Effective cooling is achieved, inter alia, by the base or cover of a cooling fluid duct being formed directly by the rear side of the semiconductor substrates or their ceramic insulating carrier. A disadvantage of this solution is that the insulating carrier or the semiconductor substrate must perform a dual function as an electrical insulator and a heat conductor. As a result, the choice of material is restricted very greatly, primarily to insulation ceramics, or compromises with regard to heat conductivity have to be accepted. Since, furthermore, the heat dissipation takes place entirely via the insulating carrier, the latter is dimensioned with a large surface area or width to increase the exchange area. On the other hand, the heat from the semiconductor elements, of small surface area, is produced very locally and, as can be demonstrated, spreads out laterally only slightly when passing through the semiconductor substrate and the insulating carrier. In this arrangement, therefore, an adequate cooling performance can be achieved only with difficulty. In addition, the cooling duct is composed relatively expensively from a plurality of materials, which moreover may have coefficients of expansion which differ from one another and, in particular, from the insulating carrier. Therefore, the cooling efficiency, thermal stability and service life of such modules become unsatisfactory.